A single-ended recuperative (SER) radiant tube heating system is disclosed in Collier U.S. Pat. No. 5,241,949. In general, an SER radiant tube heating system comprises an outer heat-resistant radiant tube having a closed forward end. Located within the outer radiant tube is an elongated inner tube which coacts with the radiant tube to define an annular exhaust passage for the flow of hot combustion products produced by a burner assembly. The burner assembly includes a nozzle which is supplied with gaseous fuel by way of a gas supply pipe. The inner tube may be divided into two portions. A recuperator portion is defined as the portion of the inner tube which extends upstream of the nozzle, and a firing portion is defined as the portion of the inner tube which extends downstream of the nozzle.
In operation, gaseous fuel and combustion air are supplied to the nozzle and are mixed therein to produce an intense flame which travels along the firing portion of the inner tube. The flame directly heats the firing portion of the inner tube which, in turn radiates heat to the radiant tube. Combustion products discharged from the forward end of the inner tube strike the closed end of the radiant tube and flow reversely in the annular passage between the two tubes to further heat the radiant tube by convection. The hot combustion products also heat the recuperator portion of the inner tube which, in turn, preheats the combustion air flowing inside the inner tube toward the nozzle.
Most conventional radiant tube heaters, however, have an unacceptable Nox emission level. Such heaters unevenly distribute air and fuel, which results in uneven combustion downstream of the nozzle. Uneven combustion creates localized areas of elevated temperature, called hot spots, near the nozzle. The hot spots not only cause undue wear, but also cause the burner to generate higher Nox emissions.
Previous SER radiant tube heating systems, such as the one disclosed in the Collier patent, have taught the use of a nozzle having combustion air ports oriented to create a spiral flow pattern. The spiral air flow improves mixing of the air and fuel, resulting in improved temperature uniformity along the length of the inner tube and, consequently, improved quality of combustion. Despite these improvements, radiant tube heating systems using such combustion air ports still develop hot spots along with inner tube, thereby limiting the maximum input for the heating system.
Flame detection in previous SER burners is also overly difficult. The flame produced in such conventional systems during low input situations is typically concentrated near the axis of the tube and only shortly downstream of the nozzle. As a result, the nozzle structure itself often blocks a direct view of the flame, rendering flame sensing ineffective. Furthermore, mixing under low input conditions is often incomplete, resulting in a flame which produces a relatively weak UV signal. Under high input conditions, combustion takes place further downstream of the nozzle. The movement of the flame makes it difficult to provide unobstructed right lines which allow a sensing device to sense the flame over a range of inputs.
Not only is flame detection difficult, but problems with flame detection in conventional heating systems also serves to overly limit the operating range of those systems. For example, a heating system may have a 10:1 fuel turndown, referred to herein as a physical turndown ratio. Conventional systems, however, are typically capable of sensing UV over a smaller range, such as a 3:1 turndown, referred to herein as a UV turndown ratio. As a result, the operating ranges of conventional systems are overly limited by the UV turndown ratio.
In addition, it has been found that, during cold start conditions, burner systems often experience varying back pressures which act to plug the flow of fuel through the gas supply pipe. The cold conditions during startup often cause less stable combustion, and therefore combustion air is consumed at continually varying rates. The unstable combustion creates periodic back pressures which act to restrict or completely plug the flow of fuel from the pipe. Conventional burners, therefore, do not consistently maintain flame during cold start operation.